The present invention refers generally to a method for making a plate package to be used in a plate heat exchanger, in particular a plate heat exchanger in the form of an evaporator, i.e. a plate heat exchanger designed for evaporation of a fluid in a refrigeration system. The refrigeration system may by way of example be an air conditioning system, a cooling system or a heat pump system. Normally, such a refrigeration system comprises in addition to the evaporator, a compressor, a condenser and an expansion valve, all of which are coupled in series. It goes without saying that the plate package may also be used for distribution of other fluids, such as steam in steam evaporators.
A typical plate heat exchanger includes a plate package, with a number of first heat exchanger plates and a number of second heat exchanger plates, which are joined to each other and arranged side by side in such a way that a first plate interspace is formed between each pair of adjacent first heat exchanger plates and second heat exchanger plates and a second plate interspace between each pair of adjacent second heat exchanger plates and first heat exchanger plates. The first plate interspaces and the second plate interspaces are separated from each other and provided side by side in an alternating order in the plate package. Substantially each heat exchanger plate has at least a first porthole and a second porthole, wherein the first portholes form a first inlet channel to the first plate interspaces and the second portholes form a first outlet channel from the first plate interspaces.
In a plate package of this kind, the plates are often brazed, bonded or welded together. However, gaskets may also be used as a sealing means between adjacent heat exchanger plates.
The fluid, i.e. the refrigerant supplied to the inlet channel of such a plate heat exchanger for evaporation is usually present both in a gaseous state and a liquid state. This is known as a two-phase evaporator. It is difficult to provide an even or optimal distribution of the fluid to the different plate interspaces in such a way that e.g. an even or optimized quantity of fluid is supplied and flows through each plate interspace. One reason for this may be that the fluid, after having passed through the expansion valve, is already partly evaporated when it enters the inlet channel, and does not remain in the state of a homogenous liquid/vapour mixture during the passage along the whole length of the inlet channel, but tends to partly separate into streams of liquid and vapour, respectively.
Uneven distribution of fluid to the different evaporation flow paths in the plate heat exchanger results in ineffective use of parts of the plate heat exchanger. Moreover, the fluid may become unnecessarily overheated. Furthermore, some channels may be flooded by liquid fluid and there is also a risk that some liquid may be present at the outlet. The latter should be avoided due to a risk of liquid entering the compressor.
In order to avoid the problem of uneven distribution of the fluid in a plate heat exchanger of the above mentioned type it has previously been suggested in SE 8702608-4 to arrange a restriction means in each passage between the inlet channel of the plate heat exchanger and each plate interspace forming an evaporation flow path for the fluid. The restriction means could be a ring or a washer provided with a hole and being arranged between adjacent pairs of the heat transfer plates around the port hole. Alternatively, the restriction means could be a pipe provided with multiple holes or apertures and being arranged in the inlet channel of the plate heat exchanger. As a further alternative it has also been suggested in SE 8702608-4 to create restriction means as an integral part of the heat transfer plates by folding the plate edge portions delimiting the inlet ports of two adjacent heat exchanger plates to abutment against each other, edge to edge. In a small area however, inlet openings are formed allowing fluid to pass into the flow paths between adjacent plates.
Plate heat exchangers provided with restriction means of the above mentioned kind give rise to several difficulties during the manufacture thereof. The use of separate rings or washers has resulted in problems with the location of the rings or washers in the correct positions when a plate heat exchanger is assembled. A restrictions means in the form of a pipe has the disadvantage that it must have a length adapted to the number of heat exchanger plates included in the plate package and it must also be correctly positioned in relation to the inlet passages leading into the flow paths between the heat exchanger plates. Folding of port edge portions of the plates has also been shown to be unpractical, depending on the fact that it is difficult to obtain well defined inlet openings leading into the plate interspaces as proposed in SE 8702608-4.
WO2010/069872A1 relates to the problem of designing a plate heat exchanger that is rigid enough to withstand the high coolant pressure that is used when using carbon dioxide as a coolant. The document provides a solution to the fact that brazed heat exchangers tend to break close the port openings if subjected to high pressures, since the tearing apart force is highest around the port openings. The document discloses a brazed heat exchanger wherein each heat exchanging plate is provided with a port skirt at least partly surrounding the port opening. As the heat exchanger plates are stacked the port skirts are overlapping one another to thereby form a pipe like configuration. The port skirts are provided with preformed holes to thereby minimize the pressure drop of the fluid during its passage through the port openings.
Yet another solution is disclosed in US 2008/0196874 in which the individual heat exchanger plates are provided with a collar surrounding the port hole. By the collars, a smooth inlet channel is formed when the heat exchanger plates are stacked to form a plate package. The sealing area in at least one of two adjacent plates may be provided with at least one narrow recess or groove forming an inlet passage allowing a flow of fluid from the inlet channel into the plate interspaces. Again, by the brazing there is a clear risk that the recesses or grooves are blocked by solder, which provides an uncontrolled and unpredictable flow pattern. Further, due to practical and economical reasons, the freedom to design and optimize the plate heat exchanger in view of a specific customer's needs is limited in terms of the number and the position of the through holes distributing the fluid.